Conventional advice is to sow parsnips now but if their reputation for poor germination is connected to being placed in cold wet soil there may be better germination by sowing in March.  ‘Avonresister’ is an early maturing variety and has very good resistance to canker, and ‘Gladiator F1’ is the worlds' first hybrid parsnip producing a heavy crop

Jerusalem artichokes and shallots can be planted now - shallots will benefit from covering with a cloche.

Last chance to plant garlic.

In trays or pots . . .

• Round seeded (as opposed to wrinkle seeded) peas - watch out for mice and perhaps start them indoors
• If suitable you can sow your broad beans in February - best sown early so the plants are well grown by the time blackfly appears. These beans are very hardy and will thrive in early spring conditions.
• Winter salads & oriental greens to plant outside in March, but still under a cloche or mini-tunnel

In trays or pots somewhere warm (germinator/warm airing cupboard etc).  Bear in mind that they will need somewhere warm & light to grow on

• Tomatoes, peppers and aubergines
• Onions from seed should be started now. They need about 15 degrees to get them going so you may be best using the windowsill in a cool room to start them off.

In a polytunnel or greenhouse . . . 

• Winter salads and oriental greens (mizuna, mibuna, mustard greens, pak choi, mispooona, komatsuna, winter varieties of lettuce, land cress)
• In warmer areas sow carrots in the tunnel for an extra early crop
• Summer cabbages such as Greyhound and Primo, as well as turnips and spinach

Bob Flowerdew even suggests trying cucumbers in January! "Seed sown right now (January) will, in most years, be cropping in late March or April and in full production for the following five months"

Feeding the soil

Root crops and legumes (peas and beans) should thrive without any additional feeding. Other crops may benefit from a dressing of manure, well rotted compost or an organic fertiliser.

Use compost at a rate of up to 2 shovels full per square metre, manure at half that rate.

Fertilisers don't help improve soil structure – so, on poorer soils it helps if, at the same time, you can add a low fertility soil improver, such as leafmould.